Traditionally, topcoat materials have been used in photolithography as anti-reflective films on the top of a photoresist. The top anti-reflective coat (TARC) materials can prevent the multiple interference of light that takes place within the photoresist layer during exposure. As a result, the critical dimension (CD) variation of the geometrical features of a photoresist pattern caused by the variation in the thickness of the photoresist film can be minimized.
To fully take advantage of the anti-reflective effect of the topcoat, the refractive index of the topcoat material (nt) should be at about the square root of the product of the refractive index of the exposure medium (nm) and the refractive index of the underlying photoresist (nr). If the exposure medium is air, as in the case for “dry” lithography, the optimal refractive index of the topcoat material (nt) should be at about the square root of the refractive index of the underlying photoresist (nr) since the refractive index of air is roughly 1.
For ease of processing, classic TARC materials are designed to be soluble in both water and aqueous base developers so that they can be applied directly from water solution and subsequently removed by the aqueous base developer during the develop stage.
Numerous topcoat materials have been developed to meet these two requirements of optimal refractive index and solubility. For example, U.S. Pat. Nos. 5,744,537 and 6,057,080 disclose aqueous-soluble TARC materials comprising a polymeric binder and a fluorocarbon compound which have nearly ideal refractive indices on the order of 1.3-1.4. U.S. Pat. No. 5,879,853 also discloses a TARC material that is removable by a wet process. U.S. Pat. No. 5,595,861 similarly discloses a TARC comprising partially fluorinated compounds which can also be water soluble. U.S. Pat. No. 6,274,295 discloses a TARC material comprising a light absorbing compound having a wavelength of maximum absorption higher than an exposure wavelength used to expose the photoresist. This TARC can also be water-soluble. Finally, U.S. Pat. No. 5,240,812 discloses a protective material for use as an overcoat film for acid catalyzed resist compositions to prevent contamination from vapors of organic and inorganic bases. While not specifically disclosed as being a TARC, the overcoat can also be water soluble.
Immersion lithography offers the potential to extend the use of optical lithography to print smaller features. In immersion lithography, air is replaced by a liquid medium such as water between the lens and the wafer. Use of a medium with an index of refraction higher than air results in a greater numerical aperture (NA), and therefore allows printing of smaller features. See “Technology Backgrounder: Immersion Lithography,” published by ICKnowledge.com at http://www.icknowledge.com, May 28, 2003. See also L. Geppert, “Chip Making's Wet New World,” IEEE Spectrum, Vol. 41, Issue 5, May 2004, pp. 29-33.
One of the technical challenges facing liquid immersion lithography is the diffusion between the photoresist components and the immersion medium. That is, during the immersion lithographic process, the photoresist components leach into the immersion medium and the immersion medium permeates into the photoresist film. Such diffusion is detrimental to photoresist performance and might result in disastrous lens damage or contamination in a 40 million dollar lithography tool. Topcoat materials can be applied on top of the photoresist layer for the purpose of eliminating diffusion of materials from the photoresist layer underneath. Preferably, the topcoat material can also act as a TARC layer.
Since water has been proposed as the exposure medium for 193 nm immersion lithography, classic water-soluble TARC materials such as those described above cannot be used as topcoats for such technology. Other commercial materials currently available either require solvents that are incompatible with semiconductor fab lines or impact the lithographic performance of the photoresist. New topcoat materials are needed to ensure the deployment of 193 nm immersion lithography necessary for manufacture of semiconductor devices at 65 nm and below design ground rules.
A serious technical challenge facing immersion lithography is the presence of bubbles and/or other small particles at the photoresist surface. The presence of small bubbles and/or particles at the photoresist surface causes printable image defects. One way to prevent this effect is through the application of a topcoat of sufficient thickness such that the bubbles and/or small particles are out of the focal plane of the lens and thus they are no longer lithographically printable. This thick coating can be described as an in situ pellicle. For purposes of this application, such a coating will be referred to simply as a pellicle. In addition, the thick topcoat material may reduce extraction of resist components into the immersion medium as compared to prior art thin topcoat materials. Since prior art immersion topcoat materials have a lower dissolution rate in developer than the underlying exposed photoresist, such topcoat films are necessarily very thin, so they can be quickly stripped with developer during the photoresist develop step. If the topcoat film is not removed quickly, the photoresist performance will be negatively impacted. Therefore, a topcoat film that is thick enough to move the surface bubbles and/or particles out of the lithography process focal plane would not be acceptable for use in immersion lithography unless the thick topcoat film could still be rapidly stripped under normal development conditions.
Thus, there remains a need for a topcoat material that is highly soluble in developer, resistant to immersion fluid, compatible with photoresist, and has desired optical properties so that it can also be used as a TARC.